1. Field of the Invention
The invention relates to a method for power level control of a display device and an apparatus for carrying out the method.
More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on displays like plasma display panels (PDP), and all kind of displays based on the principle of duty cycle modulation (pulse width modulation) of light emission.
2. Description of Prior Art
Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat colour panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology.
One important quality criterion for a video picture is the Peak White Enhancement Factor (PWEF). The Peak White Enhancement Factor can be defined as the ratio between the peak white luminance level, to the luminance of a homogeneous white field/frame. CRT based displays have PWEF values of up to 5, but present Plasma Display Panels, (PDP), have PWEF values of about 2 only. Therefore, under this aspect the picture quality of PDPs is not the best and efforts must be taken to improve this situation.
A Plasma Display Panel (PDP) utilises a matrix array of discharge cells which could only be xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame (sustain pulses). This time-modulation will be integrated by the eye over a period corresponding to the eye time response.
It is an object of the present invention to disclose a method and an apparatus for power level control which results in an increase of the Peak White Enhancement Factor.
The present invention, reports a technique that increases the PWEF of a PDP by increasing the number of available power level modes, in number and in range.
The invention starts from the reflection that for larger peak white luminance values in plasma displays more sustain pulses are necessarily required. On the other hand, more sustain pulses correspond also to a higher power consumption of the PDP. The solution is a control method which generates more or less sustain pulses as a function of average picture power, i.e., it switches between different modes with different power levels. For clarity the power level of a given mode, is defined here as the number of sustain discharges activated for a video level of 100 IRE (Institute of Radio Engineers) Therein, the relative unit 100 IRE denotes the video signal level for the full white colour. The available range of power level modes, is approximately equal to the PWEF. For pictures having relatively low picture power, i.e. a lot of pixels with relatively low luminance value, a mode will be selected which has a subsequently high power level to create the different video levels because the overall power consumption will be limited due to a great amount of pixels with low luminance value. For pictures having relatively high picture power, i.e. a lot of pixels with relatively high luminance value, a mode will be selected which has a subsequently low power level to create the different video levels because the overall power consumption will be high due to a great amount of pixels with high luminance value.
In principle the invention consists of a method for power level control in a display device having a plurality of luminous elements corresponding to the pixels of a picture, wherein the time duration of a video frame or video field is divided into a plurality of sub-fields (SF) during which the luminous elements may be activated for light emission in small pulses corresponding to a sub-field code word which is used for brightness control, characterised in that a set of power level modes is provided for sub-field coding, wherein to each power level mode a characteristic sub-field organisation belongs, the sub-field organisations being variable in respect to one or more of the following characteristics:
the number of sub-fields
the sub-field type
the sub-field positioning
the sub-field weight
the sub-field pre-scaling
a factor for the sub-field weights which is used to vary the amount of small pulses generated during each sub-field;
wherein the method comprises the steps of determining a value (AP) which is characteristic for the power level of a video picture and selecting a corresponding power level mode for the sub-field coding.
Advantageously, additional embodiments of the inventive method are disclosed in the respective dependent claims.
Contrary to CRTs, where the switching is analogue, between a continuous and in principle infinite number of modes, in PDPs the switching is discrete. By introducing an hysteresis like switching behaviour of the power level modes an oscillation between two power level modes, with perceptible differences in luminance, caused by picture noise is avoided (see claim 4 and 5).
The invention consists further in an apparatus for carrying out the inventive method. Here, the invention consists of an apparatus for carrying out the inventive method which comprises an average picture power measuring circuit, a pre-scaling unit, a sub-field coding unit and a power level control unit in which a table of power level modes and a hysteresis curve for power level mode switching control is stored.